In a treatment method based on a particle beam, a high-energy particle beam, such as a proton beam or a carbon beam accelerated up to 70% of the light velocity, is utilized. These high-energy particle beams have the following features when irradiated into a body. Firstly, almost all of irradiated particle beams stop at a position of the depth proportional to the particle beam energy raised to the 1.7th power. Secondly, the density (referred to as a dose) of energy, which is given to the path through which an irradiated particle beam passes until it stops in a body, becomes maximum at a position where the particle beam stops. A distinctive deep dose distribution curve formed along a path through which a particle beam passes is referred to as a Bragg curve. The position where the dose value becomes maximum is referred to as a Bragg peak.
A three-dimensional particle beam irradiation system is contrived in such a way that, while it scans the Bragg peak position in accordance with the three-dimensional shape of a tumor and adjusts the peak dose at each scanning position, a predetermined three-dimensional dose distribution is formed in a tumor region, which is a target preliminarily determined by an imaging diagnosis. The scanning of the position where a particle beam stops includes scanning in transverse directions (X and Y directions) which are approximately perpendicular to the irradiation direction of a particle beam and scanning in a depth direction (Z direction) which is the irradiation direction of a particle beam. In the transverse-direction scanning, there exists a method of moving a patient with respect to a particle beam and a method of moving the position of a particle beam by use of an electromagnet or the like; in general, the method utilizing an electromagnet is adopted. Scanning in the depth direction is performed only by changing the energy of a particle beam. As the method of changing energy, there exists a method of changing the energy of a particle beam by means of an accelerator and a method of inserting an energy attenuator into a path through which a particle beam passes and changing the attenuation amount of the attenuator. The method of moving the position of a beam by use of an electromagnet (referred to also as scanning) is disclosed, for example, in Patent Document 1. As shown in FIG. 2 of Patent Document 1, in a particle beam irradiation system of a conventional particle beam therapy system, as a means for moving the position of a beam spot, a scanning magnet (referred to also as a scanning electromagnet) which deflects a particle beam to the X-Y direction, which is a direction perpendicular to the traveling direction of a beam (the Z direction) is utilized.